Summary

Extremely Large ground based Telescopes (ELTs), with apertures between
25 and 100 m, are now being seriously studied by a number of groups. These
proposals have several major technological barriers to overcome, but it
seems probable that on the same timescale (~2010) as space missions such
as NGST from NASA
+ ESA
and
SPICA
(neé H2L2) from Japan (ISAS)
respectively, one or more such telescopes may be becoming operational.
Their performance, with atmospheric seeing correction by multi-conjugate
adaptive optics (MCAO;
see sidebar on linked site), will make them dominant in sensitivity
for both imaging and spectroscopy for 1 < lambda < 2.5 um and, in
the larger apertures being considered,
competitive in sensitivity
for spectroscopy with resolution R > 10,000 in the atmospheric windows
at 3-4, 8-13 and 20 um wavelength. At all these wavelengths the ELTs
would have angular resolution far superior to that offered by any alternative
facilities yet proposed for these wavelengths.

1.1 Introduction: how the ELT idea developed.

A decade ago there were approximately ten 4-m class ground based telescopes
in operation with one 10-m telescope coming into service. Today there are
perhaps 12 telescopes in the 4-m class while, remarkably, no less than
16 telescopes in the ~8-m (``VLT") class will soon be coming into
operation. In space the now-veteran 2.4-m HST continues to generate a steady
stream of dramatic results, while the proposers of its successor, the NGST,
plan to hurdle the 4-m category completely and, at 8 metres aperture, to
begin an era of space VLTs.

On the ground, meanwhile, the advent of the VLTs, and the prospect of
NGST in space, have not stopped the progress of ambition. Even in
the early '90s consideration of possible 25-m class ground-based facilities
was developing (c.f., for example, Ardeberg et al. 1993,
Owner-Petersen et al. ,1994 and references therein) and technical
studies progressing (e.g. Ardeberg
et al. 1996). By 1996
also, serious exploration of alternative technologies (Bash et al. 1996)
was producing cost estimates as low as $150M for some 25-m telescope designs,
while current science drivers and technical options were being closely
examined. Mountain (1996) set what is now a benchmark scientific objective:
to be able to secure spectra of the faintest objects in the Hubble
Deep Fields. This requires a 50-m telescope working close to
its diffraction limit. The possibility of 100-m telescopes was soon explored
(Gilmozzi et al.
1998); such a facility could perform the same service
for a large fraction of the objects expected to be seen in deep images
with the NGST.

Even more recently, the critical technique of Multi-Conjugate Adaptive
Optics (MCAO)
, originally proposed by Beckers (1988, 1989) has been demonstrated
to work (Ragazzoni et al. 2000a). This promises (c.f. Ellerbroek
& Rigaut 2000) to liberate Adaptive Optics (AO) from the limitations
of classical single-guidestar techniques, which constrain fields of view
(FOVs) to a few arcsec, across which the point-spread function (PSF) varies
greatly. MCAO has the potential to provide, at least in the important NIR
wavelength range 1 - 2.5 um, quite uniform, near-diffraction-limited,
images over FOVs of order arcminutes. The combination of MCAO with
extremely large (>20 m) telescopes promises facilities of such power as
to offer a discontinuous change in astronomical capability ``comparable
to that of the invention of the telescope itself" (Gilmozzi
et al. 1998,
section 6).